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Re: PHREEQC




Good, but you need to look at the output more carefully.


SOLUTION 1
EQUILIBRIUM_PHASES 1
Calcite 0 10
CO2(g) -2.0 10
END
SOLUTION 2
EQUILIBRIUM_PHASES 2
Calcite -1 10
CO2(g) -2.0 10
END


> The differences between these two simulations are the changes in the
saturation indices in the calcite at equilibrium. The concentrations stay
the same.


Concentrations are not the same. The above input file generates a printout
of 4 solutions: (1) an initial solution calculation for solution 1, (2) a
reaction calculation of solution 1 reacted with equilibrium_phases 1, (3)
an initial solution calculation for solution 2, and (4) a reaction
calculation of solution 2 reacted with equilibrium_phases 2. (1) and (3)
are not very interesting, they show the distribution of species in pure
water. (2) and (4) are the important sections of the output.


Here is a section of output for (2). In this problem, equilibrium_phases
contains 10 moles of Calcite and 10 moles of CO2 and we want to react
enough in solution 1 to attain a saturation index of 0 for calcite and a
partial pressure of -2 for CO2. (The log of the partial pressure is
numerically equal to the value printed for saturation index.) 10 moles is
sufficient to attain equilibrium with all but the most soluble phases, so
we expect the output to have the saturation index and partial pressure that
we specify, and indeed, the "Phase assemblage" output indicates SI=0 for
calcite and -2 (= log partial pressure) for CO2 (SIs are also in the
"Saturation Indices" section of the output). To reach this state, 1.646
mmol of calcite dissolved; the amount of calcite decreased from 10 moles to
9.998. 1.976 mmol of CO2 dissolved. The total C and Ca in solution 1 were 0
(pure water). The concentration of C is 3.6 mmol and Ca 1.6 mmol after the
equilibration has occurred. pH is ~7.3.



-------------------------------Phase
assemblage--------------------------------

                                                     Moles in assemblage
      Phase               SI log IAP  log KT    Initial      Final
Delta

      Calcite           0.00   -8.48   -8.48 1.000e+001
9.998e+000-1.646e-003
      CO2(g)           -2.00  -20.15  -18.15 1.000e+001
9.998e+000-1.976e-003

-----------------------------Solution
composition------------------------------

      Elements           Molality       Moles

      C                3.622e-003  3.622e-003
      Ca               1.646e-003  1.646e-003


----------------------------Description of
solution----------------------------

                                       pH  =   7.297      Charge balance



Here is a section of output for (4). In this problem, equilibrium_phases
contains 10 moles of Calcite and 10 moles of CO2 and we want to react
enough to attain a saturation index of -1 for calcite and a partial
pressure of -2 for CO2. So the difference is that we want to attain a
different saturation state for calcite in the resulting water. In the
"Phase assemblage" output indicates SI=-1 for calcite and -2 (= log partial
pressure) for CO2. However, to reach this state, 0.7203 (compare to 1.646)
mmol of calcite dissolved; the amount of calcite decreased from 10 moles to
9.999. 1.059 mmol of CO2 dissolved.  The concentration of C is 1.78 mmol
and Ca 0.7 mmol after the equilibration has occurred. pH is ~6.95.



-------------------------------Phase
assemblage--------------------------------

                                                     Moles in assemblage
      Phase               SI log IAP  log KT    Initial      Final
Delta

      Calcite          -1.00   -9.48   -8.48 1.000e+001
9.999e+000-7.203e-004
      CO2(g)           -2.00  -20.15  -18.15 1.000e+001
9.999e+000-1.059e-003

-----------------------------Solution
composition------------------------------

      Elements           Molality       Moles

      C                1.780e-003  1.780e-003
      Ca               7.203e-004  7.203e-004

----------------------------Description of
solution----------------------------

                                       pH  =   6.953      Charge balance



SOLUTION 3
EQUILIBRIUM_PHASES 3
Calcite 0 0
CO2(g) -2.0 10
END


> This third simulation is the again changes the saturation index of the
calcite. The concentration is the same because the default is 10 moles.


Check the output, is your statement correct?


Your next questions are what is the pH effect of (1) calcite dissolution?
(2) calcite precipitation? and (3) varying log PCO2 combined with calcite
equilibrium?


David


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled




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